Calculator inputs
Formula used
The calculator supports direct offered traffic input, or derives traffic from busy-hour calls and average holding time.
Erlang B: use this when blocked calls are cleared and do not queue.
Erlang C: use this when delayed calls wait in a queue.
For Erlang C, stable operation needs capacity greater than offered traffic.
How to use this calculator
- Select Erlang B for loss systems, or Erlang C for queued systems.
- Choose whether you want GoS, required channels, or maximum supportable traffic.
- Enter offered traffic directly, or enter busy-hour calls and average holding time.
- Provide the number of servers or channels when the chosen mode needs capacity.
- Enter a target GoS when solving inverse sizing problems.
- For Erlang C, set the waiting threshold used for service-level evaluation.
- Press the calculate button and review the result card above the form.
- Use the CSV or PDF buttons to export the solved scenario.
Example data table
| Scenario | Model | Traffic (E) | Servers | Threshold (s) | Resulting GoS |
|---|---|---|---|---|---|
| Microwave trunk group | Erlang B | 8.00 | 12 | 20 | 5.141% |
| Campus PBX busy hour | Erlang B | 15.00 | 20 | 20 | 4.559% |
| Dispatch queue | Erlang C | 10.00 | 14 | 20 | 11.165% |
| Service desk queue | Erlang C | 18.00 | 24 | 30 | 6.024% |
Frequently asked questions
1) What does grade of service mean here?
Here, grade of service is the probability that service is unacceptable during the busy hour. In Erlang B, that means blocking. In Erlang C, this page treats GoS as the probability of waiting longer than your selected threshold.
2) When should I use Erlang B?
Use Erlang B when requests are lost or rerouted immediately after finding all channels busy. It fits classic trunk sizing, radio channels, and other no-queue systems where blocked demand does not remain in line.
3) When should I use Erlang C?
Use Erlang C when delayed calls wait for service instead of disappearing. It is common for agent groups, dispatch desks, and help lines where queued demand matters more than pure blocking.
4) Why can I enter calls per hour instead of erlangs?
Traffic in erlangs is often derived from call volume and average holding time. This page converts busy-hour calls into traffic automatically, which is useful when you know call counts but not the offered traffic directly.
5) Why does Erlang C reject some inputs?
A stable Erlang C queue needs more service capacity than offered traffic. If offered traffic reaches or exceeds the number of agents, waiting time grows without bound and the basic queue formulas no longer represent a stable system.
6) What does the threshold mean in Erlang C?
The threshold is the maximum waiting time you consider acceptable. The calculator then estimates the probability a call waits beyond that limit and also reports the complementary service level achieved within that interval.
7) Is a lower GoS always better?
Usually yes, because lower GoS means fewer blocked calls or fewer long waits. However, achieving very small probabilities can require noticeably more channels or agents, so practical design balances service targets and cost.
8) Can I use this for non-telephony systems?
Yes, if your system reasonably matches the assumptions of a loss model or a queued service model. Examples include machine repair requests, server pools, dispatching resources, and other busy-hour capacity planning problems.